Warming up

What to expect from the next big report on climate change

Here are two fairly safe predictions about the climate in 2013. Unless there is a major volcanic eruption, the average global temperature will be higher than it was for all but one year in the 20th century. And towards the end of September there will be a lot of headlines about the ever greater warming yet to come.

The first is a safe bet because average temperatures have been higher than they were for all but the very hottest year in the 20th century for the past ten years, and unless a volcano cuts the amount of incoming sunlight by spreading a thin haze of particles through the stratosphere much the same can be expected in 2013. The second will come about because, at a meeting in Stockholm, the Intergovernmental Panel on Climate Change (IPCC) will release the first tranche of the latest in its series of massive “assessment reports” on climate change. This first bit will lay out the current scientific understanding of the physics, chemistry and biology of the climate and of what humans are doing to it.

Whenever such assessments are released (the previous one, the fourth in the series, came out in 2007, and the one before that in 2001), accompanying spin says that the situation looks worse than ever before. In a trivial sense this is true. The main way in which humans are changing the climate is by emitting carbon dioxide, and they are emitting ever more of it, so the climatic prospects are getting worse. But on the question of how much harm this will actually do—how bad things will look at any given level of carbon dioxide—things are unlikely to be so clear-cut.

Accompanying spin says that the situation looks worse than ever before

The climate is horrendously complex, and the more one thinks about it, the greater the horrendousness. In 1995 some academics at Carnegie Mellon University took the opinions of a number of climate researchers on the warming that the world would see if the carbon-dioxide level doubled: the range they thought likely was quite broad; the range they thought possible a lot broader; the likelihood they put on further research narrowing down the uncertainties was low. A similar study in 2010 bore out the last part of the analysis; the uncertainties were indeed little changed, though the predictions had crept up a little, with 11 of 13 experts putting the median warming between 3 and 3.5 degrees.

One of the problems is that for scientists making climate models better means getting them to take direct account of more and more of the processes involved in the climate, such as the various ways clouds work—widely seen as the single greatest source of uncertainty—or the way shifts in vegetation retard or exacerbate change. But putting in such extra wrinkles, while making the picture more complete, has so far done little to reduce uncertainties; it has, on the contrary, tended to uncover them. So the new IPCC report will give a fuller and richer sense of how the climate works, but not greater certainty on how precisely it may behave over the coming decades and centuries.

The report should nevertheless be an improvement. The scenarios for future emissions have been revamped in a way that makes more sense. There will be a fuller treatment of various aspects of cloud physics—and of the ways such physics might conceivably be changed through “geoengineering” (the IPCC has previously been quite quiet about the possibilities for such deliberate cooling technologies, but is paying them more heed this time). The previous report was irritatingly incomplete on the subject of sea-level rise, ducking the question of what melting from Greenland and Antarctica might add to the rising that is caused simply by warming up the oceans; this report should be better in that regard.

Playing Russian roulette with the planet?

The language the report uses to talk about likelihoods should be clearer, as should the links between the IPCC’s conclusions and the scientific research supporting them. Tightened procedures mean that mistakes such as the erroneous claim that the Himalayas would lose all their glaciers, which marred the previous assessment, are unlikely in the 2013 report. But recent reforms to the IPCC’s procedures will do little to change its tendency to focus on the areas where there is greater consensus, avoiding the uncertainties which, though unpalatable for scientists, are important to policy

Take that key question of the warming that can be expected for a doubling of carbon dioxide. Last time round the IPCC said it was likely to be between 2 and 4.5 degrees—meaning there was a 66% chance of its sitting in that range. That left 17% chances that it would be less, or more. As Robert Socolow, an energy and climate-policy analyst at Princeton, has pointed out, these extremes (in particular the high one) need to be dealt with more explicitly, because their consequences would be great.

And their likelihoods, though low, are not that low: 17% is the chance of blowing your head off in a game of Russian roulette.